An MCF is comprised of a plurality of cores extending along a fiber axis direction (which is a direction coinciding with the longitudinal direction of the MCF and extending along the fiber axis passing a center of a cross section of the MCF perpendicular to the longitudinal direction), and a common cladding covering each of these cores. Each of the cores optically functions as an optical waveguide. Since the MCF has a larger number of cores per unit cross-sectional area than a single-core fiber does, it is suitable for transmission of large volumes of information.
As a method for fusion splice between two MCFs, Patent Literature 1 discloses an example of implementing optical axis alignment between the MCFs by making use of marker portions provided in the respective MCFs. Specifically, in a fusion splice operation, first, while monitoring the side faces of the two MCFs as splicing objects, each of the MCFs is rotated around the fiber axis and the two MCFs are checked as to positions after rotation (hereinafter referred to as “rotational positions”) of the cores and marker portion varying with the rotation. When agreement is confirmed between monitor patterns of the two MCFs (or between arrangements each consisting of the cores and marker portion in the MCFs, which are displayed on a monitor screen during monitoring their side faces), an alignment work is carried out by horizontally/vertically moving the core positions while keeping the end faces of the two MCFs facing each other. After the alignment, the end faces of the two MCFs as splicing objects are subjected to fusion splice.